Study Of Laser-Induced Breakdown Spectroscopy Technology Used For Soil Component Detection

In this paper, the spectral characteristics of laser-induced soil plasma were studied.Thelaser spectrum analytical system is composed of a SQ-II Nd：YAG laser(0～700mJ),a planegrating spectrograph and a spectral collection and data processing system. National standardsoil samples were used as ablating target.The main subjects are as the following:1. The influence of laser pulse energy and defocusing of laser beam on plasma radiation.The influence of laser output energy （100-500mJ）and on the radiation intensity wasstudied. The results show that both the line intensity and signal-to-background ratio of plasmaemission spectrum can be enhanced under the optimized condition that the laser energy was200mJ. The quality of spectrum was further improved after the laser beam used to excite thesample was defocused properly. When the defocusing position was+6mm,the spectral linesintensity of element Mg、Al、K and Fe increased about46%、63%、59%and45%than thatwithout defocusing respectively. The spectral signal-to-background ratio increased about11%、31%、35%and38%respectively. Thus proved that proper laser energy and properdefocusing of laser beam can strengthen the radiation of laser-induced soil plasma distinctly.2. Enhancement effect of NaCl additive on laser-induced soil plasma radiationThe enhancement effect of laser output energy ranging from100mJ to500mJ and NaCladditive ranging from0%to20%on the radiation intensity of plasma was studied. Theexperimental results show that both the line intensity and signal-to-background ratio can beenhanced under the optimized condition that the laser energy was200mJ. Furtherimprovement of spectrum quality had been observed with proper amount NaCl additive intosoil sample. For instance, after10%NaCl was added, the spectral lines intensity of elementFe、Ba、Al and Ti increased around37%，32%，32%and70%than that without additive;spectral signal-to-background ratio increased about33%，26%，16%and38%; while theablation mass, electron temperature and electron density of plasmas went up14%,9.6%and26%,respectively. It was thus clear that adding NaCl additive to soil sample was a effectivemeasure for the enhancement of spectrum quality. 3. The influence of laser pulse repetition rate on laser-induced soil plasma spectrumLaser pulse repetition rate was directly related to the evaporation，atomization andexcitation process of laser towards sample，and also properties of plasma. Emission spectrumquality of plasma under different laser pulse repetition rate of5Hz，10Hz，15Hz wasinvestigated. By comparing the spectral intensity and signal-to-background ratio,it was seenthat laser with high repetition rate was beneficial for obtaining higher quality atomicemission spectrum. When there was no CsCl additive, the spectral intensity andsignal-to-background ratio of elements Al，Fe and Ti under the condition that the laser pulserepetition rate was15Hz increased around40%、78%、66%and62%、30%、9%than that thelaser pulse repetition rate was5Hz. When the CsCl additive content was10%, the spectralintensity and signal-to-background ratio of elements Al，Fe and Ti under the condition that thelaser pulse repetition rate was15Hz increased around44%、108%、100%and10%、40%、66%than that the laser pulse repetition rate was5Hz. It was demonstrated that higher laserpulse repetition rate could further improve the spectral quality of plasma.Thus was essential toreducing detection limit of spectral analysis and enhancing the detection sensitivity.4. Analysing quantitatively the content of heavy metal in the soil by using Laser-inducedBreakdown Spectroscopy.National standard soil samples(GBW07411)was used as analysis sample. Underoptimum conditions, emission spectrum of soil plasma was measured. Standard AdditionMethod was used to analyse in quantity for a toxic heavy metal element–Cr in thesample.The experimental results showed that when spectral line of internal standard elementwas used to eliminate the adverse effect casued by the fluctuation of experimentalconditions,satisfactory results of quantitative analysis were achieved.The feasibility of LIBSused for soil component detection was demonstrated.